Chemical vapor deposition of titanium from titanium tetrachloride and hydrocarbon reactants

ABSTRACT

A new process for depositing titanium metal layers via chemical vapor deposition is disclosed. The process provides deposited titanium layers having a high degree of conformality, even in trenches and contact openings having aspect ratios greater than 1:5. The reaction gases for the improved process are titanium tetrachloride and a hydrocarbon gas, which for a preferred embodiment of the process is methane. The reaction is carried out in a plasma environment created by a radio frequency source greater than 10 KHz. The key to obtaining titanium metal as a reaction product, rather than titanium carbide, is to set the plasma sustaining electrical power within a range that will remove just one hydrogen atom from each molecule of the hydrocarbon gas. In a preferred embodiment of the process, highly reactive methyl radicals (CH 3 —) are formed from methane gas. These radicals attack the titanium-chlorine bonds of the tetrachloride molecule and form chloromethane, which is evacuated from the chamber as it is formed. Titanium metal deposits an a wafer or other substrate that has been heated to a temperature within a preferred range of 200-500° C.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of application Ser. No. 08/581,765,filed Jan. 2, 1996, pending.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to chemical vapor deposition reactions,integrated circuit manufacturing and, more particularly, to methods fordepositing titanium metal layers on in-process integrated circuits.

2. Background of Related Art

Deposited titanium metal layers are being used with increasing frequencyin integrated circuits. One important application involves the formationof contact structures within a dielectric layer. The processing ofwafers for the manufacture of integrated circuits commonly requires thatcontact openings be etched through an insulative layer down to implantor diffusion regions in a semiconductor layer to which electricalcontact is to be made. Titanium metal is then deposited over a wafer sothat the surface of each exposed implant/diffusion region is coated. Thetitanium metal is eventually converted to titanium silicide. A silicideis a binary compound formed by the reaction of silicon with the metal atelevated temperatures. The titanium silicide layer serves as anexcellent conductive interface at the surface of the implant/diffusionregion. A titanium nitride barrier layer is then deposited, coating thewalls and floor of the contact opening. The contact plugs are formed bydepositing a tungsten or polysilicon layer via chemical vapordeposition. In the case of tungsten, the titanium nitride layer providesgreatly improved adhesion between the walls of the opening and thetungsten metal. In the case of the polysilicon, the titanium nitridelayer acts as a barrier against dopant diffusion from the polysiliconlayer into the diffusion region.

Deposited titanium metal layers are also used as an underlayment foraluminum alloy layers deposited on interlevel dielectric layers. Thetitanium and aluminum alloy layer stack is etched to form interconnectlines within the integrated circuit. The titanium metal layer not onlyprovides increased resistance to electromigration of aluminum atoms, butalso provides improved adhesion of the aluminum alloy layer to thedielectric layer as compared with an aluminum alloy layer without thetitanium underlayment.

Two principal techniques are presently available for creating thintitanium films: deposition via reactive sputtering of a titanium targetand chemical vapor deposition. When topography is present, reactivesputtering results in titanium films having poor step coverage. Althoughcollimated sputtering improves the coverage on trench floors, it doesnot help coverage on vertical surfaces. In fact, as trench aspect ratios(the ratio of depth to width) exceed 4 or 5 to 1, the deposition rate atthe bottom of the trench is minimal because of the buildup of depositedmetal at the mouth of the trench. As the mouth of the trench narrowsduring the deposition process, the comers of the trench floor receiveincreasingly less deposited material. Because of the step-coverageproblem, sputter-deposited films are limited primarily to underlaymentlayers on relatively planar surfaces. Another problem related tocollimated sputtering is that the collimator grid dramatically slows thedeposition rate and must be cleaned frequently.

Chemical vapor deposition processes have an important advantage oversputter deposition techniques in that the deposited layers have muchhigher conformality (i.e., uniform thickness on both horizontal andvertical surfaces), layers of any thickness may be deposited, and thedeposition rate does not slow with time (as with collimated sputtering).This is especially advantageous in modem ultra-large-scale-integration(ULSI) circuits, where minimum feature widths may be smaller than 0.3 μmand trenches and contact openings may have width to depth aspect ratiosof 1:5 or more. In U.S. Pat. No. 5,173,327, a chemical vapor depositionprocess for titanium is disclosed. Titanium tetrachloride and hydrogengas are admitted to a chemical vapor deposition chamber in which asubstrate (i.e., semiconductor wafer) has been heated to about 400° C.Titanium tetrachloride molecules are adsorbed on the substrate surfaceand react with hydrogen with the following chemical equation:TiCl₄+2H₂=Ti+4HCL. The deposition rate of this reaction can be enhancedby striking a radio-frequency plasma in the deposition chamber. Becausethe diatomic hydrogen molecule is relatively difficult to ionize, theflow rate of hydrogen gas into the deposition chamber must beconsiderably greater than that for titanium tetrachloride. The low ratioof titanium tetrachloride molecules to hydrogen molecules is notconducive to high deposition rates. In addition, as the aspect ratio oftrenches and contact openings increases, step-coverage worsens due tothe limited amount of titanium tetrachloride that is adsorbed toward thebottom of the trenches and contact openings. Although the aforementionedtitanium deposition process is satisfactory for many applications, thepresent invention aims at providing a chemical vapor deposition processfor titanium having increased conformality and more rapid depositionrates.

SUMMARY OF THE INVENTION

This invention is embodied in a new process for depositing titaniummetal layers via chemical vapor deposition. The process providesdeposited titanium layers having a high degree of conformality, even intrenches and contact openings having aspect ratios greater than 1:5. Thereaction gases for the improved process are titanium tetrachloride and ahydrocarbon gas, which for a preferred embodiment of the process ismethane. The chemical reaction is as follows:

TiCl₄+4CH₄=Ti+4Ch₃Cl+2H₂

The reaction is carried out in a plasma environment created by a radiofrequency AC source greater than 10 KHz. The standard FCC-assignedfrequencies of 400 KHz and 13.56MHz are entirely satisfactory. The keyto obtaining the proper reaction products (i.e., titanium metal ratherthan titanium carbide) is to set the plasma sustaining electrical powerwithin a range that will break just one hydrogen bond from thehydrocarbon gas. In the case of methane, highly reactive methyl radicals(CH₃—) are formed. These radicals attack the titanium-chlorine bonds ofthe tetrachloride molecule and form chloromethane, which is evacuatedfrom the chamber as it is formed. In the case of other hydrocarbongases, highly reactive alkyl radicals are formed. The alkyl radicalsattack the titanium tetrachloride and form an alkyl chloride gas whichis evacuated from the chamber.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a diagrammatical view of a pressurized, chemical vapordeposition chamber showing an arrangement for the introduction of thereactants of the new chemical vapor deposition process for titaniummetal.

DETAILED DESCRIPTION OF THE INVENTION

This new process for depositing titanium metal layers via chemical vapordeposition provides titanium layers having a high degree ofconformality, even in trenches and contact openings havingwidth-to-depth aspect ratios greater than 1:5. The reaction gases forthe improved process are titanium tetrachloride (TiCl₄) and ahydrocarbon gas, which for a preferred embodiment of the process ismethane (CH₄). Other hydrocarbon gases having the general formulasC_(n)H_(2n+2), C_(n)H_(2n) and C_(n)H_(2n−2) are also potentialcandidates.

The chemical reaction of the preferred embodiment of the process, whichemploys methane gas, is as follows:

TiCl₄+4CH₄=Ti+4CH₃Cl+2H₂

The reaction is carried out in a plasma environment created by a radiofrequency AC source greater than 10 KHz. The standard FCC-assignedfrequencies of 400 KHz and 13.56 MHz are entirely satisfactory forcarrying out the reaction. The key to obtaining the proper reactionproducts (i.e., titanium metal rather than titanium carbide) is to setthe plasma sustaining electrical power within a range that will breakjust one hydrogen bond of the hydrocarbon gas molecules. In other words,the power setting is maintained at a level that is greater than thefirst ionization energy, but less than the second ionization energy forthe selected hydrocarbon gas or gases. A power range of about 20 to 100watts meets this requirement when methane is selected as the reactanthydrocarbon gas. It may be necessary to adjust both the power settingand the AC source frequency when other hydrocarbon gases are used. Bybreaking just one hydrogen bond in the methane molecule, highly reactivemethyl radicals (CH₃—) are formed. These radicals attack thetitanium-chlorine bonds of the tetrachloride molecule and formchloromethane, which is evacuated from the chamber as it is formed. Inthe case of other hydrocarbon gases, highly reactive alkyl radicals areformed. The alkyl radicals attack the titanium tetrachloride and form analkyl chloride gas which is evacuated from the chamber.

The process will now be described in reference to the diagrammaticrepresentation of the plasma enhanced chemical vapor deposition (PECVD)chamber of FIG. 1. Although the following description of the processrepresents what the inventors believe is the preferred embodiment of theprocess, the process may be practiced in either cold-wall or hot-wallplasma-enhanced chemical vapor deposition chambers, with or withoutpremixing of the reactants. Furthermore, although the invention isdirected to a technique for depositing conformal titanium layers for usein the manufacture of integrated circuits, the process is alsoapplicable to the deposition of titanium on substrates other thansemiconductor wafers. Referring now to FIG. 1, a titanium tetrachloride(TiCl₄) source gas is produced by heating liquid TiCl₄. The gas phaseTiCl₄ is admitted into a premixing chamber 3 through control valve 1 anda hydrocarbon gas such as methane CH₄ is admitted into the premixingchamber 3 through control valve 2. Following the premixing of the gasphase TiCl₄ and the CH₄ in premixing chamber 3, the premixed gases areadmitted to the deposition chamber 4. Optionally, the gas-phase TiCl₄ orthe hydrocarbon gas, or both, may be mixed with a carrier gas such asargon (Ar) or helium (He). For example, helium gas may be bubbledthrough the heated TiCl₄ to further enhance the complete gasification ofthat reactant, while argon gas might be added to the hydrocarbon gas inorder to dilute that reactant and/or set a desired deposition pressure.As a further option, liquid TiCl₄ may be converted to a fine spray ormist by a liquid injector (not shown). The mist is then passed through avaporizer chamber (also not shown) en route to the deposition chamber.The flow rate of the hydrocarbon gas in standard cubic centimeters perminute (scc/m) should be four to about 1,000 times that for the TiCl₄.Within the deposition chamber, a semiconductor wafer 5 is heated byconvection from substrate holder 6 (such as a graphite or alumina boat)that in turn is heated to a preferred temperature of 200 to 500° C. viahalogen lamps 7. The walls of the chamber are maintained at atemperature which will prevent condensation of titanium tetrachloridethereon. By maintaining the wafer at a temperature that is considerablyhigher than the chamber walls, deposition of titanium metal on the wallscan be minimized. The temperature of the chamber walls should bemaintained within a range of about 50-400° C., and optimally within arange of about 100-200° C.

Still referring to FIG. 1, the premixed gas combination of TiCl₄ and thehydrocarbon gas enters deposition chamber 4 through shower head 15. Aradio-frequency voltage, supplied by radio-frequency generator 8, isapplied between substrate holder 6 and deposition chamber 4, thusforming alkyl radicals from the hydrocarbon gas in the space above thewafer 5. The TiCl₄ is adsorbed on the surface of the wafer 5, and alkylradicals react with the adsorbed TiCl₄ molecules to deposit a uniformlythick titanium metal layer on all exposed surfaces of the wafer. As hightemperatures favor the formation of inorganic halides as opposed totitanium metal, the reaction temperature is maintained within a range ofabout 200° C. to 500° C. Although the desired reaction will occur at apressure within a range of about 2 to 100 torr, a preferred range isdeemed to be about 2 to 5 torr. A constant deposition pressure withinthat preferred range is monitored and maintained by conventionalpressure control components consisting of pressure sensor 9, pressureswitch 10, air operating vacuum valve 11 and pressure control valve 12.The alkyl chloride gas given off as a byproduct of the reaction, whethermethyl chloride (CH₃Cl) or an alkyl chloride, and the carrier gases (ifcarrier gases are used) pass through particulate filter 15 and escapethrough exhaust vent 14 with the aid of a Roots blower 13 to completethe process.

It is to be understood that although the present invention has beendescribed with reference to a preferred embodiment, variousmodifications known to those skilled in the art may be made to theprocess steps presented herein without departing from the scope andspirit of the invention as hereinafter claimed.

What is claimed is:
 1. A chemical vapor deposition process fordepositing a titanium metal layer on a substrate comprising the stepsof: (a) placing the substrate within a plasma-enhanced depositionchamber; (b) admitting titanium tetrachloride and at least onehydrocarbon gas into said chamber; (c) igniting a plasma which ismaintained at a power level sufficient to form radicals therein; and (d)heating the substrate to a temperature sufficient to induce the radicalsto scavenge chlorine atoms from the titanium tetrachloride leavingtitanium metal atoms on a surface of the substrate.
 2. The process ofclaim 1, wherein the substrate is heated to a temperature within a rangeof 200-500° C.
 3. The process of claim 1, wherein said at least onehydrocarbon gas is selected from a group consisting of the compoundsC_(n)H_(2n+2), C_(n)H_(2n) and C_(n)H_(2n−2).
 4. The process of claim 1,wherein said at least one hydrocarbon gas is an alkane having fewer thanfive carbon atoms per molecule.
 5. The process of claim 4, wherein saidat least one hydrocarbon gas is methane.
 6. The process of claim 1,wherein said at substrate is mounted on a heated susceptor, and saidsubstrate is heated by thermal conduction from said heated susceptor. 7.The process of claim 1, wherein said titanium tetrachloride is mixedwith a carrier gas selected from a the group consisting of helium, argonand hydrogen.
 8. The process of claim 7, wherein the titaniumtetrachloride is introduced into said carrier gas in a bubblerapparatus.
 9. The process of claim 7, wherein a liquid injector sprayssaid titanium tetrachloride, the spray then being passed through avaporizer.
 10. The process of claim 1, wherein said at least onehydrocarbon gas is mixed with a carrier gas selected from a groupconsisting of helium and argon.
 11. The process of claim 1, whereinnon-depositing reaction products are removed from the deposition chamberas the process proceeds.
 12. The process of claim 11, wherein an alkylchloride gas is a principal reaction product.
 13. The method of claim 1,wherein said deposition chamber is a cold wall deposition chamber, thewalls of which are maintained at a temperature within a preferred rangeof 100-200° C. in order to prevent condensation of the titaniumtetrachloride thereon.
 14. The method of claim 1, wherein saiddeposition chamber is a hot wall deposition chamber.
 15. The method ofclaim 1, wherein said plasma is produced with a radio frequency source.16. The method of claim 15, wherein said radio frequency source operatesat a power setting within a range of 20 to 100 watts.
 17. The method ofclaim 15, wherein said radio frequency source operates at a frequencygreater than 10 KHz.
 18. The method of claim 1, wherein said depositionchamber is maintained at a pressure within a range of 2 to 100 torr. 19.The method of claim 1, wherein said deposition chamber is maintained ata pressure within a preferred range of 2 to 5 torr.
 20. The method ofclaim 1, wherein the titanium tetrachloride and the at least onehydrocarbon gas are premixed before being admitted to the depositionchamber.
 21. The method of claim 20, wherein a ratio of hydrocarbon gasto titanium tetrachloride in the premixture is between four and onethousand to one.
 22. The method of claim 1, wherein said substrate is asemiconductor wafer.
 23. A chemical vapor deposition process fordepositing a titanium metal layer on a semiconductor wafer comprisingthe steps of: (a) placing the semiconductor wafer within aplasma-enhanced deposition chamber; (b) admitting titanium tetrachlorideand at least one hydrocarbon gas into said deposition chamber; (c)igniting a plasma which is maintained at a power level that is greaterthan a first ionization energy, but less than a second ionizationenergy, of the at least one hydrocarbon gas forming hydrocarbonradicals, and (d) heating the semiconductor wafer to a temperaturesufficient to induce the hydrocarbon radicals to scavenge chlorine atomsfrom the titanium tetrachloride molecules forming chlorinatedhydrocarbon molecules and leaving titanium metal atoms on a surface ofthe substrate.
 24. The method of claim 23, wherein the depositionchamber is maintained at a pressure within a range of 2 to 10 torr. 25.The method of claim 23, wherein said power level comprises a radiofrequency source operating at a power setting within a range of 20 to100 watts and at a frequency greater than 10 KHz.
 26. The method ofclaim 23, wherein the titanium tetrachloride and the at least onehydrocarbon gas are premixed prior to their admission to the depositionchamber, having a ratio of methane gas to titanium tetrachloride in thepremixture being between four and one thousand to one.
 27. The method ofclaim 23, wherein the at least one hydrocarbon gas is methane.
 28. Theprocess of claim 23, wherein said at least one hydrocarbon gas isselected from a group consisting of the compounds C_(n)H_(2n+2),C_(n)H_(2n) and C_(n)H_(2n−2).